A known gear device includes an eccentric cam having a support portion which is eccentrically arranged relative to a rotational axis and rotatably actuated, an internally toothed gear provided coaxially with the eccentric cam, and an externally toothed gear supported by the eccentric cam and meshed with the internally toothed gear.
For example, JPH05-26304A (hereinafter referred to as Patent reference 1) discloses a construction in which an output shaft is connected to an externally toothed gear via an Oldham's coupling. By fixing the internally toothed gear, according to the construction disclosed in Patent reference 1, a rotation of the eccentric cam can be reduced on the basis of a difference of the number of teeth between the internally toothed gear and the externally toothed gear and outputted.
Notwithstanding, whereas a speed reducer which is downsized and having high speed reduction ratio can be provided by adopting the foregoing construction, there is a drawback that the construction is assumed to be complex by the application of the Oldham's coupling. Further, because the speed reducer attains the high speed reduction ratio, the large load is applied between the meshed gear teeth. Thus, in order to transmit a high torque, face widths (lengths along tooth trace) of the internally toothed gear and the externally toothed gear need to be extended to increase the meshing dimension between the meshed gear teeth. The axial dimension, or axial length is assumed to be long according to the foregoing construction, which leaves room for improvement.
A need thus exists for a gear device which is not susceptible to the drawback mentioned above.